feat(frontend): preserve unsaved form state across 503 error transitions

[fpindej]

Problem

When the backend goes down while a user has unsaved form input (e.g. profile name, phone number, bio), invalidateAll() triggers the 503 error page which destroys all client-side state. After the backend recovers and the page auto-reloads, the user has to re-enter everything they typed.

Discovered during PR #244 testing — the 503 auto-recovery flow works correctly but is destructive to in-progress work.

Proposed approach

Persist dirty form fields to sessionStorage before the 503 transition and restore them after recovery:

1. Create a $lib/state/form-persistence.svelte.ts utility (e.g. useFormPersistence(key, fields))
2. On 503 detection (before invalidateAll()), snapshot dirty fields to sessionStorage
3. On page mount, check for a saved snapshot and restore field values
4. Clear the snapshot after successful form submission or on explicit discard

Open questions

• Should this be opt-in per form, or automatic for all forms?
• Should it use sessionStorage (tab-scoped) or localStorage (persistent)?
• How long should snapshots be retained? TTL-based cleanup?

Affected areas

• src/lib/components/profile/ProfileForm.svelte
• src/lib/components/settings/ChangePasswordForm.svelte
• src/lib/components/admin/ — create/edit dialogs
• Any future forms

Context

• Related PR: fix(frontend): fix session expired redirect and SSR crash on login page #244 (fix/login-session-expired)
• The 503 transition is triggered by backend-monitor.ts middleware calling invalidateAll()